This application relates to methods of producing flowable, small-particle size, high molecular weight, water-soluble polyacrylamide from larger size particles, improved methods for making solutions of the polyacrylamide, and their uses for soil conditioning.
Water-soluble polyacrylamide (PAM) and other water soluble polymers are used as soil conditioners because they help form and protect soil aggregates by binding to clay particles in the soil. Among the benefits, this property helps to control wind and water erosion, improve water infiltration and retention, improve soil aeration, and inhibit crusting or sealing. One use is in forestry, wherein granules of PAM are mixed into soil into which seedlings are planted. Water-soluble PAM is a long-chain molecule, which is distinguished from an insoluble cross-linked form of PAM that is used for different purposes. This disclosure relates to the water-soluble variety of PAM, which will also be referred to herein simply as PAM.
Water-soluble PAM with the most desirable properties for soil conditioning has a molecular weight of about 15-22 million a.u. (atomic units), and is about 20% anionic. Water-soluble PAM of this molecular size is commercially available in granule sizes of about +60, xe2x88x9225 mesh (between 250 and 600 xcexcm), and, more typically, essentially not smaller than +40 mesh ( greater than 450 xcexcm) sizes. PAM of this type, which comprises an anionic linear copolymer of acrylamide and sodium acrylate, is available under the trade name FLOBOND A30 from Chemtall, Inc. of Riceboro, Ga. The water-soluble granules appear to be aggregates of many molecules and have a very irregular shape, as shown in FIG. 1A. Under an optical microscope, flat crystal faces are visible, and the particles appear translucent. Some small portions of the granules, e.g. dark areas in FIG. 1A, appear transparent. The granules are probably not entirely clear because of included fractures and other defects, which will scatter light.
Severe barriers exist to more widespread use of water-soluble PAM in soil. When dry granules of PAM of standard commercial sizes are applied to agricultural soil at reasonable economic rates, the soil typically is nonuniformly conditioned because the dry granules are too far apart to fully condition soil. For example, when 10 pounds (4.5 kg) of the standard size water-soluble PAM are mixed into an acre of soil to a uniform depth of six inches (15 cm), each pound (0.45 kg) of soil will contain on average only about 22 particles of water-soluble PAM. Each ounce (28.4 grams) of soil would contain only about 1-2 particles.
Applying water-soluble PAM in solution is more effective than dry granule application because it produces a more uniform spatial distribution of the PAM in the soil. Therefore, solution application requires less PAM than does application of granules for favorable results. Another benefit of using a solution of PAM is that it can be applied to soil through standard irrigation systems. However, the commercially available PAM granules have a slow and low solubility in water. These solubility charateristics make it difficult to obtain sufficiently concentrated stock solutions of water-soluble PAM to be economically useful when injected into irrigation lines. Full solution time for standard sized granules can be an hour or more but many dealers and manufacturers say that it is best to allow the water-soluble PAM to sit overnight in water to fully dissolve in a stock solution. This is too long to be practical for most large scale agricultural purposes.
Generally, stock solutions of around 2,000 to 3,000 parts per million (ppm) by weight (mg/liter) water-soluble PAM in water are the most concentrated that can be conveniently made with conventional procedures. When fertilizer salts are included, somewhat higher concentrations, e.g. up to about 12,000 ppm, are possible. The presence of fertilizer solutes will also help PAM go into solution somewhat more rapidly. For example, solution concentration can be improved by disssolving PAM in various fertilizer salt solutions, as described in U.S. Pat. No. 4,797,145, to Wallace, et al. However, the solution rate generally remains slower than desirable because of the large particle sizes. An hour or more is typically required to achieve the higher concentrations even with the use of fertilizer salts.
Co-application of dry PAM with some divalent calcium helps the water-soluble PAM to bridge with or react with clay to add stability to soil aggregates, as described xe2x80x9cNeed for Solution or Exchangeable Calcium and/or Critical EC Level for Flocculation of Clay by Polyacrylamides,xe2x80x9d by Wallace and Wallace, in xe2x80x9cProceedings: Managing Irrigation-Induced Erosion and Infiltration with Polyacrylamide,xe2x80x9d Univ. Idaho Misc. Pub No. 101-96, pp. 59-63, 1996. This enhances the soil conditioning value. To achieve this, solution-grade gypsum of xe2x88x92200 mesh particle size ( less than 75 xcexcm), which is typically composed mostly of calcium sulfate dihydrate, can be applied to soil before addition of water-soluble PAM, or gypsum and water-soluble PAM are applied together in solution after each is dissolved separately. But gypsum also has a low solubility, which limits its use.
Using water-soluble PAM in solution for liquid application to soil entails high handling costs. It generally requires bulk equipment that is not easily portable to fields because large volumes of stock solution are needed for large fields. The PAM granules of the size used commercially take too much time to go into solution. Because low PAM concentrations of stock solution are the rule, large amounts of stock solution are needed for each application. Dissolving standard-sized polymers requires considerable experience. Training is often required to become proficient in getting water-soluble PAM particles into solution. When directions are not followed, failed applications result. If the PAM is not completely dissolved, particles of PAM tend to clump together in agglomerations. These large undissolved clumps make using solutions of PAM in sprinkler irrigation systems very difficult, as the clumps tend to clog the sprinkler lines and nozzles.
One approach to the time and concentration problems has been to use mechanical devices that meter the water-soluble PAM into a stream of irrigation water. A residence time in a tank of one hour or more before applying the solution to fields is realistic and common. For example, U.S. Pat. No. 5,450,985, to Meuleman, discloses a device that delivers dry water-soluble PAM into a canister and from there into an irrigation water stream, such as an irrigation ditch or canal, after a time period. This system does not produce PAM solutions for injection into sprinkler irrigation systems. U.S. Pat. No. 5,580,168, to Alireza, et al., discloses a venturi system for injecting water-soluble PAM first into a dispersion tank and then into an aging tank, which is further agitated before injecting the stock solution into an irrigation system. Solution time for both systems is much too slow for convenience. The size of the granules of water-soluble PAM that are used commercially is too large to allow faster solution times.
U.S. Pat. No. 5,548,020, to Santini et al., discloses an alternative procedure for putting water-soluble PAM into irrigation lines. A 30 percent concentrated emulsion product is prepared with kerosene or oil. The flow rate of this product is relatively slow, which decreases its usefulness for sprinkler irrigation systems. Also, this form of PAM flows into water like a semi-stiff string that requires considerable mechanical turbulence by a machine to put into solution. The kerosene or oil adds expense and appears to decrease the effectiveness of the soil conditioning properties of the PAM, and they are environmentally undesirable. The water-soluble PAM in the concentrated 30 percent solution-emulsion is considerably more expensive than granular or powder forms of water-soluble PAM to further detract from any advantage it may have.
Water-soluble polymers also have uses in drilling for oil, and oil drillers have developed various methods for putting the polymers into solutions. The polymer solutions produced for oil drilling have a high viscosity, which is undesirable for irrigation. The oil drilling solutions are typically produced without any accurate measuring of the amounts of the ingredients, which is not appropriate for agriculture. For these reasons the oil drilling industry methods are not applicable to the aqueous requirements for solution of water-soluble PAM for agricultural use.
The time needed for dissolution of water-soluble PAM can be decreased by using smaller particle size PAM, however there is no adequate bulk supply of suitable small particle PAM. Attempts have been made to grind PAM into smaller sizes, such as xe2x88x92100 mesh ( less than 150 xcexcm). However, PAM ground in this way loses many of its desirable properties. For example, the ground PAM is irregularly shaped, as shown in the electron micrograph in FIG. 1B. When observed with an optical microscope, the particles are entirely opaque. Most surfaces do not appear to have clean, flat faces, as with the larger size commercial grade particles. The particles tend to form clumps in water, which are difficult to dissolve. Many particles remain undissolved in water even after an hour or more. The solution formed from the dissolving particles is less viscous than a solution produced with a like concentration of PAM which was not first ground. This suggests that many of the large polymeric molecules have been broken by the grinding process. The ground PAM also has poor flow characteristics, as would be desirable in a metering system. This is probably due to the irregular shape of the ground particles, and their tendency to form clumps. In addition, the process for grinding the larger water-soluble PAM granules typically includes freezing the granules, which adds to the cost and complexity of the procedure.
Small quantities of small particle size water-soluble PAM can be obtained by screening the generally large-granule PAM from commercial sources. This is a laborious process which does not yield sufficient quantities to be commercially cost effective. As shown in the micrograph in FIG. 1C, the screened fines are very similar in appearance to the larger size, commercial grade PAM. When observed with an optical microscope, the particles are translucent, which may be due to fractures included in the particles. About 40-60% of the particles appear to have areas which are transparent. When added to plain water, the particles do not disperse well. Some of the particles appear to clump together. Many particles remain undissolved and visible, even after ten minutes of stirring or agitation. The screened fines also have poor flow characteristics.
In one aspect, the invention provides a material comprising dry, flowable, water-soluble polyacrylamide particles that are characterized by particle sizes of about xe2x88x92100 mesh ( less than 150 xcexcm), and prefereably +270 mesh ( greater than 53 xcexcm). The polyacrylamide particles are essentially all soluble in plain or distilled water within about 10 seconds or less, to concentrations of up to about 0.5% by weight. The polyacrylamide particles essentially consist of molecules having a molecular weight of at least about 15 million a.u. At least about 90% of the polyacrylamide particles are further characterized by being transparent.
The polyacrylamide particles may be combined with one or more members of the group consisting of a calcium salt, a fertilizer, and gypsum. The calcium salt can include a member of the group consisting of calcium nitrate, calcium thiosulfate, and calcium chloride. The fertilizer may include potassium salts, ammonium salts, and mixes thereof which may also include calcium salts. The gypsum is preferably characterized by particle sizes of about xe2x88x92200 mesh, and more preferably about xe2x88x92325 mesh or smaller ( less than 30 xcexcm).
The material may be produced according to a process that includes the following steps: providing dry granules of water soluble polyacrylamide characterized by a particle size in a range of about +60, xe2x88x9225 mesh, and consisting essentially of molecules having a molecular weight of at least about 15 million a.u.; subjecting the granules to rapid pressure increases and decreases; and breaking up the granules into smaller particles with the pressure increases and decreases.
The invention also provides a method of reducing the mean particle size of dry granules of water-soluble polyacrylamide (PAM) that consist essentially of molecules having a molecular weight of at least about 15 million a.u. The method includes subjecting the granules to rapid pressure increases and decreases, and breaking up the granules into smaller particles with the pressure increases and decreases. The smaller particles are characterized by being flowable, and essentially consist of molecules having a molecular weight of at least about 15 million a.u. The smaller particles are preferably characterized by a particle size of about xe2x88x92100 mesh.
In one embodiment, subjecting the granules to the rapid pressure increases includes providing a particular type of mill to subject the particles to the rapid pressure changes. The mill includes a housing characterized by a first end having an input adapted to feed the granules into the housing, a second end having an output adapted to remove the smaller particles, and longitudinally extending internal sides that form longitudinally extending interior corners where they meet. A rotor assembly within the housing is characterized by a rotatable shaft extending longitudinally through the housing between the first and second ends, and a plurality of rotors coupled to the shaft for rotation therewith. Rotors of the plurality of rotors each include a rotor plate having a polygonal-shaped peripheral edge forming a plurality of apices, and vanes on a side of the rotor plate which each extend approximately radially from an apex. There is an orifice plate positioned between adjacently located pairs of the plurality of rotors. Each orifice plate extends inwardly from the internal sides of the housing to a central aperture which provides an orifice around the shaft. The method includes feeding the granules of water-soluble PAM into the housing while rotating the rotors at a speed sufficient to cause the granules to flow in an alternating outward and inward flow around peripheral edges of the rotor plates and through the orifices, for example, at a rotation rate of at least about 3000 RPM. Pressure on the granules increases and then decreases each time one of the vanes pass closely by a side of the housing. Pressure on the granules also increases as the granules flow towards each orifice and decreases as the granules pass out of each orifice. Preferably, the mill further includes circumferentially spaced members located proximate each of the rotors and extending inwardly from the corners of the housing toward the rotors. Pressure on the granules rapidly increases and then decreases each time one of the vanes pass closely by one of the members. The granules flow through the housing in a Coanda flow, substantially without high angle impacts on the rotor assembly, the orifice plates or the interior sides of the housing.
The invention further provides a method of producing an aqueous solution of water-soluble polyacrylamide (PAM) with a concentration of at least about 5 grams per liter and up to about 15 grams per liter or even more. This method includes providing a monovalent or divalent cation salt solution, and preferably a calcium salt solution, and adding particles of the PAM, which are characterized by a particle size of about xe2x88x92100 mesh and preferably +270 mesh, to the calcium salt solution such that the particles are essentially all dissolved within about 10 seconds.
This method can include one or more of the following features: The ratio of calcium:PAM is between about 0.5 and 2. The calcium salt includes a member of the group consisting of calcium nitrate, calcium thiosulfate, calcium sulfate, and calcium chloride. Adding the PAM particles includes stirring or agitating the solution while adding the PAM particles. The salt solution can further include a fertilizer.
According to yet another aspect of the invention, an aqueous stock solution for soil treatment includes water-soluble polyacrylamide (PAM) with a concentration of at least about 5 grams per liter and a calcium salt. The solution has a calcium to PAM ratio that is at least about 0.5. The calcium:PAM ratio is preferably about 1.0 to about 32. The calcium salt preferably includes a member of the group consisting of calcium nitrate, calcium thiosulfate, calcium sulfate, and calcium chloride.
In still another aspect, the invention provides a method of soil conditioning that includes providing an aqueous stock solution comprised of water-soluble polyacrylamide (PAM) with a concentration of at least about 5 grams per liter and calcium with a calcium:PAM ratio of at least about 0.5, mixing the stock solution with additional water to make a diluted solution, and applying the diluted solution to an area of soil. Applying the diluted solution can include spraying the diluted solution through one or more nozzles of an irrigation system. The ratio of calcium to PAM (Ca:PAM) in the stock solution is preferably between about 0.5 and 32. The diluted solution can further include at least one of a fertilizer and gypsum.
The invention also provides a method of soil conditioning. The method includes the steps of providing a conditioning material that includes dry, flowable, water-soluble polyacrylamide (PAM) particles that are characterized by particle sizes of about xe2x88x92100 mesh and preferably +270 mesh, wherein the PAM particles are essentially all soluble in water within about 10 seconds or less, and applying the conditioning material to an area of soil. The conditioning material can further include a calcium salt, gypsum particles, a fertilizer, or another soil conditioner. Applying the conditioning material can include mixing the conditioning material with the soil such that each pound of the soil mixed with the conditioning material includes at least about 8,000 of the PAM particles.
Advantages of the invention include the following:
High quality, small particle size, water-soluble PAM can be produced in large quantities without grinding or freezing. The particles produced by this method flow very well, do not tend to clump together, and easily and quickly go into solution, even in plain tap or distilled water. The small particles retain all the soil conditioning properties of the larger size, commercial grade PAM particles. Because of their small size, they can be dispersed in the soil more thoroughly than the larger size particles. Particles can be made in discrete size ranges that can be blended with gypsum or other soil conditioners of similar particle size. These particles have a lower water content than ground particles, which makes it more efficient on a per pound basis, and therefore lower cost, for shipping and handling.
Water-soluble PAM is easily and near instantaneously put into solution. Conventional procedures using the larger size PAM granules do not do this, even with the use of fertilizer salts. Relatively high concentrations of water-soluble PAM in stock solutions are obtained when mixing small particle size PAM with divalent or monovalent cation salt solutions to exceed the levels possible with conventional procedures. The process of making solutions of water-soluble PAM is much simplified compared with conventional procedures. Failures are avoided.
When used with calcium solutions, the effectiveness for soil improvement of a given amount of water-soluble PAM is at least twice that obtained with conventional methods for solution preparation of water-soluble PAM.
Erosion control with water-soluble PAM for furrow irrigation is made more simple and more effective than conventional procedures. Lower concentrations of PAM in solution are needed for furrow irrigation.